feat: implement ControlTower TUI for cluster and host monitoring

Add complete TUI application for monitoring Kubernetes clusters and host
systems. Features include:

Core features:
- Collector framework with concurrent scheduling
- Host collectors: disk, memory, load, network
- Kubernetes collectors: pods, nodes, workloads, events with informers
- Issue deduplication, state management, and resolve-after logic
- Bubble Tea TUI with table view, details pane, and filtering
- JSON export functionality

UX improvements:
- Help overlay with keybindings
- Priority/category filters with visual indicators
- Direct priority jump (0/1/2/3)
- Bulk acknowledge (Shift+A)
- Clipboard copy (y)
- Theme toggle (T)
- Age format toggle (d)
- Wide title toggle (t)
- Vi-style navigation (j/k)
- Home/End jump (g/G)
- Rollup drill-down in details

Robustness:
- Grace period for unreachable clusters
- Rollups for high-volume issues
- Flap suppression
- RBAC error handling

Files: All core application code with tests for host collectors,
engine, store, model, and export packages.

internal/collectors/host/load_test.go

@@ -0,0 +1,48 @@
+package host
+
+import (
+	"testing"
+	"time"
+
+	"tower/internal/model"
+)
+
+func TestParseProcLoadavgFirst(t *testing.T) {
+	v, err := parseProcLoadavgFirst("1.23 0.70 0.50 1/123 4567\n")
+	if err != nil {
+		t.Fatalf("unexpected err: %v", err)
+	}
+	if v < 1.229 || v > 1.231 {
+		t.Fatalf("expected 1.23, got %v", v)
+	}
+	if _, err := parseProcLoadavgFirst("\n"); err == nil {
+		t.Fatalf("expected error")
+	}
+}
+
+func TestDesiredLoadPriority(t *testing.T) {
+	p, w := desiredLoadPriority(3.99)
+	if p != "" || w != 0 {
+		t.Fatalf("expected none")
+	}
+	p, w = desiredLoadPriority(4.0)
+	if p != model.PriorityP2 || w != 120*time.Second {
+		t.Fatalf("expected P2/120s")
+	}
+	p, w = desiredLoadPriority(6.0)
+	if p != model.PriorityP1 || w != 120*time.Second {
+		t.Fatalf("expected P1/120s")
+	}
+}
+
+func TestUpdateSustainedWorksForLoadToo(t *testing.T) {
+	now := time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC)
+	p, since := updateSustained(now, "", time.Time{}, model.PriorityP2)
+	if p != model.PriorityP2 || !since.Equal(now) {
+		t.Fatalf("expected set")
+	}
+	p2, since2 := updateSustained(now.Add(10*time.Second), p, since, model.PriorityP2)
+	if p2 != model.PriorityP2 || !since2.Equal(since) {
+		t.Fatalf("expected unchanged")
+	}
+}